The relationship between HCV core protein (HCcAg) processing and the structural composition and morphogenesis of nucleocapsid-like particles (NLPs) produced in Pichia pastoris cells was studied. At early stages of heterologous expression, data suggest that HCcAg (in the P21 form) was transported soon after its synthesis in the cytoplasm into the nucleus. HCcAg assembly into nucleocapsid-like particles with 20-30 nm in diameter took place primary in the cell nucleus. However, at later stages, when P21 and P23 forms were co-detected, data suggest that new assembly of nucleocapsid particles containing P21 possibly occurs at ER membranes and in the cytoplasm. This is the first report showing that structured HCV NLPs composed of P21 core protein assemble primary in the nucleus of P. pastoris yeast. (C) 2003 Elsevier Inc. All rights reserved.